1. Field of the Invention
This invention relates generally to controllable electronic attenuators, and more specifically, to digitally programmable attenuators or amplitude modulators capable of providing predetermined, selectable, discrete magnitudes of electronic signal attenuation in response to control signals. The invention has particular utility in the area of monolithic microwave application circuitry and will be described in connection with such utility, although other utilities are contemplated.
Present day electronic devices often incorporate controllable attenuator devices and/or components for varying the amount of resistance applied to electronic signals passing therethrough. Such attenuators are used in, inter alia, automatic gain control circuits, position locating systems, telephone systems, television systems, and microwave circuit applications.
Electronically controllable solid state attenuators for use at microwave frequencies sometimes employ PIN diodes arranged in a variety of network configurations. Circuitry for providing controlled bias to the diodes is generally used in such devices to cause the diode network to generate the desired magnitude of signal resistance. Such PIN diode-based attenuators are capable of outstanding high-frequency performance, but consume an undesirably large amount of electrical power, and additionally, are not easily integrated into monolithic microwave-application circuitry.
Other devices for providing selectable attenuation incorporate high frequency field effect transistors (FETs), such as, e.g. gallium arsenide metal semiconductor FETs, arranged in a variety of network configurations (which may include other circuit elements, e.g., discrete resistors, among others). These devices operate by applying control signals to the gates of the transistors to adjust or select the overall attenuation level of the device. The characteristic parameters of the device's individual component elements (for example, the depths of the transistors' undepleted channels and the volume distribution uniformity of the resistors, inter alia, which respectively, determine the transconductance and resistance characteristics of the transistors and resistors) determine the overall attenuation characteristics of the attenuator. Thus, in order to function properly, conventional attenuators must be fabricated with particular attention being paid to precise control of the parameters of the component elements (transistors, resistors, and the like). This precision is difficult to achieve, mainly because element parameters tend to vary with ambient temperature, device age, and, most importantly, manufacturing anomalies.
Conventional manufacturing typically produce device anomalies (some of which have been described above) in which the actual parameters of the device's component elements may vary quite widely and unpredictably from desired, nominal parameters. Usually, the actual parameters are distributed closely around the nominally desired (planned) values, but deviations of .+-.30% or more from nominal are not uncommon. This phenomenon can have the undesirable consequence that the series arm resistance (at a given current) through a given bit-branch of the attenuator may be higher than nominally expected while the shunt arm resistance thereof may be lower than nominal. This disadvantageously may cause the actual magnitudes of attenuation of a particular attenuator device to vary unpredictably from those expected. This effect may be exacerbated by ambient temperature change.
Conventional methods for mitigating the aforementioned problems (resulting from manufacturing process anomalies) involve the use of high precision (low tolerance) component elements and/or pre-tested component elements. The latter technique essentially involves careful prior examination of component elements to ascertain the elements' characteristics, and then, to incorporate into the attenuator only those elements having exactly the desired characteristics. Unfortunately, implementation of the aforesaid techniques is quite costly, and therefore, can add greatly to overall fabrication costs of the conventional attenuator device.
Examples of conventional attenuator devices are disclosed in: Hopfer, U.S. Pat. No. 4,438,415 (1984); Kamikawara, U.S. Pat. No. 4,683,386 (1987); Golio et al, U.S. Pat. No. 5,001,524 (1991); Cooper et al, U.S. Pat. No. 5,049,841; McGrath and Pratt, "An Ultra Broad Band DC-12 GHz 4-Bit Gallium Arsenide Monolithic Digital Attenuator," Gallium Arsenide IC Symposium Digest, (March 1991), pp 247-250; Khabbaz et al, "DC-20 GHz MIMIC Multi-Bit Digital Attenuators With On-Chip TTL Control," 13th Annual Gallium Arsenide IC Symposium Technical Digest 1991, pp. 239-242; Ali et al, "Low-loss, High-power, Broad Band Gallium Arsenide MIMIC Multi-Bit Digital Attenuators With On-Chip TTL Drivers," 13th Annual Gallium Arsenide IC Symposium Technical Digest 1991, pp 243-246; Pengelly, "Using Gallium Arsenide MIMICS as Control Devices," MSN, (April 1989), pp 18-28; and, Gupta et al, "A 0.05-to-14-GHz MMIC 5-Bit Digital Attenuator," Gallium Arsenide IC Symposium (April 1987), pp 231-234.